Throughout the world, considerable oil reserves are locked in the form of tar sands, also called bitumen sands. For example, the Athabasca tar sands deposit, located in northeastern Alberta, Canada, is the largest of the four major Alberta deposits and contains oil reserves exceeding 146 billion cubic meters over a total area of 32,000 square kilometers. Another such tar sands deposit exists in the Tar Sand Triangle located in a triangularly shaped area between the Dirty Devil River and the Colorado River in southeastern Utah. The Tar Sand Triangle deposit contains reserves of 12-16 billion barrels of oil in place and covers an area of approximately 518 square kilometers. However, the fact that the oil, in the form of bitumen, is intimately mixed with sand, water, sand silt, complicates its efficient extraction therefrom.
Various methods have been proposed to separate the bitumen product from the tar sands as a single component. In one method, the bitumen separated from the sands is coked to produce coker distillate which may be later refined in accordance with conventional refinery practice. In the alternative, it has been proposed that the raw tar sands be treated in a retort in either a moving or fluid bed to produce a coker distillate in which the coke which deposits on the sand is burned to provide process heat.
However, the foregoing processes have their disadvantages in that during coking, the distillate is cracked. While cracking may be desirable for obtaining economic yields, there is usually some degradation of the distillate quality.
One attempt to overcome these disadvantages is disclosed and claimed in U.S. Pat. No. 2,871,180. The method described in this patent for separating crude oil from bituminous sands in a deasphalted oil enriched layer and an asphaltene enriched layer is to provide an aqueous pulp of the sands into a vertical extraction zone. A low molecular weight paraffinic hydrocarbon (propane) is then introduced into the extraction zone at a level below the point of introduction of the aqueous bituminous sand pulp.
Essentially, the low molecular weight paraffinic hydrocarbon flows upwardly through the extraction zone while the heavier aqueous bituminous sand pulp flows downwardly. These opposing upward and downward flows result in the formation of a deasphalted oil and solvent phase (i.e., the product phase), an asphaltenes phase diluted with a lesser portion of the solvent, a water phase, and a substantially oil-free sand phase, said phases having increasing specific gravities in the order presented. The phases are then taken off for further treatment. However, this process presented several economic disadvantages that limited its use and commercial applicability.
Conventionally, the hot water extraction process, which avoids some of the disadvantages presented by the above methods, is utilized in the recovery of bitumen from the sand and other material in which it is bound. After the bitumen is recovered, it is then treated to obtain oil products therefrom. One such example of this process is disclosed by U.S. Pat. No. 5,626,743, which is incorporated herein by reference.
In the prior art water extraction process, tar sands are first conditioned in large conditioning drums or tumblers with the addition of caustic soda (NaOH) and water at a temperature of about 85.degree. C. The tumblers provide means for steam injection and positive physical action to mix the resultant slurry vigorously, causing the bitumen to be separated and aerated to form a bitumen froth.
The slurry from the tumblers is then screened to separate out the larger debris and passed to a separating cell where settling time is provided to allow the slurry to separate. As the slurry settles, the bitumen froth rises to the surface and the sand particles and sediments fall to the bottom. A middle viscous sludge layer, termed middlings, contains dispersed clay particles and some trapped bitumen that is not able to rise due to the viscosity of the sludge. Once the slurry has settled, the froth is skimmed off for froth treatment and the sediment layer is passed to a tailings pond. The middlings is often fed to a secondary flotation stage for further bitumen froth recovery.
U.S. Pat. No. 5,626,793 discloses a modified prior art water extraction process termed the hydrotransport system. In this system, the tar sands are mixed with water and caustic soda at the mine site and the resultant slurry is transported to the extraction unit in a large pipe. During the hydrotransport, the tar sands are conditioned and the bitumen is aerated to form a froth. This system replaces the manual or mechanical transport of the tar sands to the extraction unit and eliminates the need for tumblers.
The bitumen froth from either process contains bitumen, solids, and trapped water. The solids that are present in the froth are in the form of clays, silt, and some sand and contains about 60% by weight bitumen, which is in itself composed of about 10 to 20% by weight asphaltenes, about 30% by weight water, and about 10% by weight solids. From the separating cell, the froth is passed to a defrothing or deaerating vessel where the froth is heated and broken to remove the air. Typically, naphtha is then added to solvate the bitumen thus reducing the density of the bitumen and facilitating separation of the bitumen from the water by means of a subsequent centrifugation treatment. The centrifuge treatment first involves a gross centrifuge separation followed by a series of high-speed centrifuge separations. The bitumen collected from the centrifuge treatment usually contains about 5 wt % water and solids and can be passed to the refinery for upgrading and subsequent hydrocracking. The water and solids released during the centrifuge treatment are passed to the tailings pond.
The very nature of bitumen renders it difficult to process. This is because bitumen is a complex mixture of various organic species comprising of about 44 wt % white oils, about 22 wt % resins, about 17 wt % dark oils, and about 17 wt % asphaltenes (Bowman, C. W. "Molecular and Interfacial Properties of Athabasca Tar Sands". Proceedings of the 7.sup.th World Petroleum Congress. Vol. 3 Elsevier Publishing Co. 1967).
When bitumen is treated using the conventional naphtha dilution and centrifugation extraction process, considerable problems are encountered. The reason for this is actually two-fold: Firstly, the naphtha diluted bitumen product can contain up to 5 wt % water and solids. Secondly, the naphtha diluent solvates the bitumen as well as the unwanted and dirty asphaltenes contained in the bitumen froth. Because hydrocracking requires a homogeneous feed which is very low in solids and water, the naphtha diluted bitumen product cannot be fed directly to the hydrocracker. In order to utilize the naphtha diluted bitumen product, it must first be coked to drive off the naphtha solvent and drop out the asphaltenes and solids. Unfortunately, this coker upgrading represents an enormous capital outlay and also results in a loss of 10-15% of the bitumen initially available for hydrocracking.
One way to avoid the problems presented by the naphtha dilution of the bitumen is to use a different solvent such as a paraffinic diluent. However, the use of a paraffinic diluent results in the precipitation of a major proportion of asphaltenes from the diluted bitumen. Therefore, when the paraffinically diluted bitumen is fed to the centrifugation system, the precipitated asphaltenes may tend to "plug up" the centrifuges which results in increased maintenance due to the necessity of shutting down and cleaning the fouled centrifuges. The increased centrifuge maintenance therefore results in reduced throughput and unsatisfactory process economics. Furthermore, centrifugation equipment is highly capital and maintenance intensive even during smooth operation.
The tailings produced via the conventional extraction process present further problems. The tailings in the tailings pond are largely a sludge of caustic soda, sand, water, and some bitumen. During the initial years of residence time, some settling takes place in the upper layer of the pond, releasing some of the trapped water. The water released from the ponds can be recycled back into the water tar sands treatment process. However, the major portion of the tailings remains as sludge indefinitely. The sludge contains some bitumen and high percentages of solids, mainly in the form of suspended silt and clay.
The tailings ponds are costly to build and maintain, and the size of the ponds and their characteristic caustic condition creates serious environmental problems. In addition, environmental concerns exist over the large quantity of water which is required for the extraction and which remains locked in the tailings pond after use.
It is known that sludge is formed in the initial conditioning of the tar sands when caustic soda attacks the sand and clay particles. The caustic soda causes the clays, such as montinorillonite clays, to swell and disperse into platelets that are held in suspension and form the gel-like sludge. Since such sludge inhibits the flotation of the bitumen froth in the extraction process, lower grade tar sands containing large amounts of expanding clays cannot be treated satisfactorily using the conventional water caustic soda process.
Therefore, the need exists for an extraction process which would not require the use of caustic soda in the tar sands conditioning process, which would result in a reduction in the production of sludge and therefore an increase in the water available for recycling and a decrease in the sheer volume of tailings present in the tailings ponds. It would also be highly desirable to avoid to use of naphtha based solvents for bitumen extraction and preclude the necessity of coker upgrading of the bitumen product prior to hydrocracking. It would also be desirable to avoid the use of centrifuges with paraffinically diluted bitumen and the inherent asphaltenes plugging of such centrifuges by utilizing a non-capital intensive process which can efficiently treat a diluted bitumen containing precipitated asphaltenes while maintaining a high throughput, low maintenance, and improved process economics.
Processes to utilize alternative conditioning reagents other than caustic soda have been proposed. U.S. Pat. No. 4,120,777 and U.S. Pat. No. 5,626,743 disclose two such processes. While the former utilizes soluble metal bicarbonates in place of caustic soda, the latter teaches the use of mixtures of sodium and potassium bicarbonates in the presence of calcium and magnesium ion sources. The aim of both of these patents is to avoid the use of caustic soda in the hot water tar sands conditioning process and therefore reduce clay dispersion and subsequent sludge formation.
U.S. Pat. No. 4,349,633 avoids entirely the use of a conditioning reagent in the tar sands conditioning process and instead teaches the use of a suspension of oxidase-synthesizing hydrocarbon metabolizing microorganisms to facilitate the separation or release of bitumen from the sand, clays, and water in the tar sands.
However, such processes have not been adopted by the industry due to the fact that they substantially increase the cost of bitumen extraction from tar sands due to the higher cost of reagents employed. Furthermore, such processes often result is lower tar sands conditioning rates and thus adversely affect product throughput. Finally, although such processes may avoid the production of sludges and their inherent problems, none of the prior art addresses the problem of coker upgrading of naphtha diluted bitumen or the centrifuge plugging resulting from paraffinically diluted bitumen.